This invention relates generally to a process for the production and recovery of hydrocarbon chemical feedstocks and hydrocarbon fuels from renewable plant sources using whole plants and extracts from whole plants. This invention in particular relates to the rapid and direct conversion of such materials by a process requiring hydropyrolysis in the gas phase.
Considerable evidence in the literature suggests that products initially formed during thermal decomposition of carbonaceous materials are largely in the liquid molecular weight range and that these products decompose, crack to gases and recombine at a rapid rate to form refractory products such as coke the longer they are subjected to the thermal decomposition conditions. Lifetime of the liquid products is also short, as is indicated by available evidence. Therefore, in order to maximize liquid yields from such decompositions, it is desirable to limit the time during which the products initially formed are subject to the decomposition conditions. Thus a low residence time of the carbonaceous material in the thermal decomposition zone and a high rate of decomposition are desirable. Similarly, thermal decomposition of carbonaceous materials in a reducing atmosphere and at low pressures facilitates the escape of volatile products from the carbonaceous material and from one another and minimizes the tendency to recombine. Also, it is generally recognized that in thermal decomposition of carbonaceous materials rapid quenching of the decomposition reactions and/or reaction of the liquid products with a stabilizing material such as hydrogen maximizes production of the liquid products. If rapid quenching does not take place or if sufficient stabilizing material is not available, some of the free radical decomposition products will polymerize to form unreactive char.
Thermal decomposition of carbonaceous materials to obtain hydrocarbon products can be effected by direct heating in the presence or absence of oxygen. The presence of oxygen causes the reactor product to contain greater or lesser amounts of carbon oxides. For example, Brink, et al., U.S. Pat. No. 3,639,111 teaches a method and apparatus for distilling organic wastes at an elevated temperature and in the presence of a controlled amount of oxygen insufficient for complete combustion, the elevated temperature being above a critical temperature zone, thereby reducing or cracking gases from the material to hydrogen, carbon monoxide and methane. Brink, et al., U.S. Pat. No. 3,718,446 teaches the pyrolysis of organic materials from pulping operations in the presence of limited oxygen at a sufficiently high temperature (of 800.degree. C. to 1,200.degree. C. or higher) and for a sufficient length of time (of from 1 to 30 seconds) to prevent recombination reactions and produce stable products such as phenols, hydrogen, carbon monoxide, carbon dioxide and methane. Anderson, U.S. Pat. No. 3,729,298 teaches a process for disposing of carbonaceous refuse by thermally decomposing it in a shaft furnace with temperatures on the order of 3000.degree. F. and simultaneously producing a fuel or synthesis gas primarily containing over 50% carbon monoxide by combusting the char and hydrogen. A gas containing at least 40% oxygen is fed into the furnace to create a thermal driving force in excess of 1600.degree. F. A low rate of oxygen introduction is taught to maintain a reducing atmosphere in the hearth to prevent overoxidation of the char to CO.sub.2 and oxidation of the metallic components of the refuse although the process can be operated under mildly oxidizing conditions.
If an oxygen source is not used to maintain direct combustion, an indirect method of heating typically is used which reduces formation of carbon oxides but requires other sources of energy input. Heat transfer, and therefore the efficiency of the process, can be accordingly diminished. For example, Grannen, et al., U.S. Pat. No. 3,843,457 teaches a process for microwave pyrolysis of organic materials to recover vaporizable organic compounds such as organic acids and aldehydes from nominally solid organic wastes by comminuting the wastes and mixing them with a gas stream at a pressure substantially less than atmospheric. The gases are preferably reducing gases, particularly hydrogen. The comminuted wastes are subject to microwave discharge which effects molecular decomposition and the vaporized components are thereby removed from the gas stream. Fleming, U.S. Pat. No. 4,002,438 teaches a method and a device for the flash pyrolytic conversion of organic materials into gaseous or liquid fuels comprising methane, hydrogen, ethane with some light oil fractions in a single self-contained vessel wherein problems of clogging, coke formation, and sludge formation are substantially avoided by use of a mixture of dense, hard abrasion-resistant material which is recycled with recycled product gas and combustion air. Choi, et al., U.S. Pat. No. 4,078,973 teaches a closed loop pyrolysis process for organic solid wastes wherein the heat is supplied by inert particles which are heated in a separate combustion zone. The residence time during pyrolysis is generally less than 10 seconds. The pyrolysis temperature is between 600.degree. F. and the introduction temperature of the inert particles. The pyrolysis zone can be between 600.degree. F. to about 2,000.degree. F. The carrier gases are oxygen-free. The products are carbon-containing char, pyrolytic oils of an oxygenated nature and gases primarily of the oxides of carbon and light hydrocarbons. Garrett, et al., U.S. Pat. No. 4,153,514 teaches a process for recovery of chemical values from waste solids wherein shredded waste solids of a 0.25 inch maximum dimension are intermixed with hot char and a carrier gas and passed through a pyrolysis zone under turbulent conditions at temperatures of from 300.degree. F. to 2000.degree. F. The maximum particle size is critical because larger sizes do not provide the high rate of heat transfer essential to the process.
Processes have been disclosed for recovering liquids from carbonaceous solids and lower boiling liquids from higher boiling liquid hydrocarbons which involve a rapid decomposition of the carbonaceous material in the presence of hydrogen and at a low pressure and a rapid quenching of the decomposition reaction. However, rapid decomposition at low pressure and rapid quenching entails the problems of feeding the material into the reactor and removing it readily.
In particular, Greene, U.S. Pat. No. 3,997,423 discloses a process of producing carbonaceous tars from liquid or crushed solid carbonaceous material comprising (1) introducing carbonaceous material into a reactor; (2) adding hot hydrogen to the carbonaceous material in the reactor; (3) reacting the hydrogen and carbonaceous material for a period of from about two milliseconds to about two seconds at a temperature of about 400.degree. C. to 2,000.degree. C. and at a pressure between atmospheric and 250 psia.; and (4) quenching the mixture within the reactor, with the total residence time for steps (2) and (3) varying from about two milliseconds to about two seconds. The patentee states that the heat-up rate of the carbonaceous material is in excess of 500.degree. C. per second.
Greene, U.S. Pat. No. 4,012,311 discloses a process which is similar to the process of Greene, U.S. Pat. No. 3,997,423 and in which the decomposition reaction takes place at a pressure between atmospheric and 450 psia.
Pelofsky et al., U.S. Pat. No. 4,003,820 disclose a process which is similar to the process of Greene, U.S. Pat. No. 3,997,423, and in which the decomposition reaction takes place at a higher pressure between 500 and 5,000 psig.
Although Pelofsky et al., U.S. Pat. No. 4,003,820 and Greene, U.S. Pat. No. 4,012,311 do disclose in general terms an additional step in which the carbonaceous material is pretreated with hydrogen prior to being decomposed, such patents do not disclose the conditions of such pretreatment.
Furthermore, none of Greene, U.S. Pat. Nos. 3,997,423; 4,012,311; and Pelofsky, U.S. Pat. No. 4,003,820 disclose a suitable method for rapidly introducing the carbonaceous material into the reactor. These patents disclose only that, in order to overcome the reactor pressure, both the carbonaceous material and the incoming hydrogen must be fed into the reactor at a pressure exceeding that of the reactor. Rapid passage of the carbonaceous material into and through the reactor is essential if a short decomposition time and a commercially acceptable, high through-put of carbonaceous material is to be achieved.
One suitable method for rapidly introducing the carbonaceous material into the decomposition zone involves entraining the carbonaceous material in a stream of compressed gas and instantaneously expanding and accelerating this stream as it passes through a restricted area into the decomposition zone. A similar technique is employed in a method for disintegrating coal solids as disclosed in Yellott, U.S. Pat. No. 2,515,542. Such technique not only serves to introduce the carbonaceous material rapidly into the decomposition zone but also permits the volatile fragments and radicals which form in the interior of the carbonaceous material to move rapidly away from the carbonaceous material and from one another.
Avco Everett Research Laboratory, Inc. has in very general terms disclosed to various people in the industry a coal gasification technique utilizing a two-stage gasifier. In the first stage, char is burned with oxygen to generate heat. The combustion gases from this combustion are then fed to a pyrolyzer through a converging-diverging nozzle. A large pressure drop is maintained across the nozzle. The combustion gases are accelerated to sonic conditions in the converging section of the nozzle, resulting in a cooling of the gases. Coal and steam are fed or aspirated into the stream of combustion gases at or slightly upstream of the throat of the nozzle. The mixture is then accelerated to supersonic flow in the diverging section of the nozzle and discharges into the pyrolyzer as a confined jet. As the gas velocity decreases from supersonic flows to subsonic flow in the pyrolyzer, a shock occurs which results in rapid heating of the coal, leading to the rapid formation of volatile material in the coal. Many of the volatiles are believed to be free radicals which are stabilized by the steam, thus preventing soot formation. Argon, carbon monoxide, helium and nitrogen have also been studied as stabilization gases. The residence time of the reaction mixture in the pyrolyzer is about 40 milliseconds.